Deprecate / remove the "delete" icon. Deleting nodes / links can now be

Deprecate / remove the "delete" icon.  Deleting nodes / links can now be
performed through the right-button-menu; alternatively, selected nodes
can be deleted using the keyboard shortcut (delete key).

NOTE: this and previous patch have slightly broken the "undo" functionality,
we will deal with this later when cross-canvas linking code will be
completed.

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